Time interval measurements between the occurrence of two signals must frequently be made. Many time interval measurement circuits have the ability to measure both positive and negative time intervals between the two signals, i.e., one of the signals is preselected as indicating a reference point in time, and if the other signal occurs before or after that signal, a negative or positive time interval would be measured.
The ability to measure both positive and negative time intervals presents an ambiguity problem when the time intervals occur repetitively. This is illustrated with respect to FIG. 1. In this example, both start and stop channel signals are pulse trains. Two time intervals, one positive and one negative, are able to be measured depending upon whether the start or stop channel is used to "arm" the measurement circuitry. Arm in this sense is used to indicate that the circuitry will measure the time interval between the next occurrence of start and stop channel events following the arming of the time interval circuitry.
Typically, the arming signal is derived from either the start or stop channel event. If the arming occurs at point 101, the time interval measured will be from point 121 to point 122. Note that the time interval which will be measured is a positive time interval, since the start event has occurred before the stop event. If the arming of the measurement circuitry occurs at point 102, the corresponding time interval measured will be from point 122 to point 123. Note that this is a negative time interval, since the stop event has occurred before the start event. Therefore, if armed randomly, the measurement circuitry will switch between measuring the interval between points 121 and 122 and the time interval between 122 and 123. Assume that both signals are pulse trains 100 nanoseconds in repetition interval and the time interval between point 121 and 122 is 30 nanoseconds, and the time interval between point 122 and point 123 is 70 nanoseconds. In that case, the display would flicker between showing a positive 30 nanosecond measurement and showing a negative 70 nanosecond measurement. Note that this problem is further complicated if the start and stop channel signals are drifting with respect to each other on repetitive measurements. In that case, even if the arming signal was consistently provided at point 101, there would still be an alternation between the positive and negative measurements because the signals 121 and 122 would not consistently be occurring in the same order. In accordance with the preferred embodiment of the present invention, one of the two input signals is chosen as the initial source of the arming signal. Thereafter, control circuitry comprising a phase detector controls the subsequent arming of the measurement circuitry after the initial selection has been made.